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fix(sky): translucency-as-opacity + sky fog floor + additive fog-skip

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Three retail-faithful sky/weather composite fixes (one cohesive commit
because they touch the same per-Surface flag plumbing path).

1. Surface.Translucency is OPACITY, not (1 - opacity).
   Retail D3DPolyRender::SetSurface at 0x59c7a6 (decomp 425255-425260)
   computes `curr_alpha = _ftol2(translucency × 255)` and writes that
   directly as vertex.color.alpha. ACViewer (TextureCache.cs:142) and
   WorldBuilder (ObjectMeshManager.cs:1115) both use `1 - translucency`
   and are wrong by the same misread. Cloud surface 0x08000023 has
   Translucency=0.25; under the old (1-x) formula opacity was 0.75,
   making clouds 3× too bright vs retail. Flipped to use translucency
   directly. Gated on the Translucent flag (0x10) so non-Translucent
   surfaces (which carry Translucency=0 in the dat) keep opacity 1.0
   instead of going invisible.

2. Sky fog re-enabled with a "fog floor" mitigation.
   Disabled 2026-04-24 because Dereth sky meshes are authored at radii
   1050-1820m while storm-keyframe FogEnd is ~400m, which would saturate
   the entire dome to flat fogColor and destroy stars/moon/dome texture.
   Retail visibly DOES fog its sky, mechanism still un-pinned. Workaround:
   clamp `vFogFactor` to a minimum of SKY_FOG_FLOOR=0.2 so the dome shows
   AT LEAST 20% raw texture even at extreme distances. Tuned via dual-
   client visual comparison; preserves stars/moon while letting the
   horizon haze visibly in low-FogEnd keyframes.

3. Additive sky surfaces skip fog entirely.
   Retail D3DPolyRender::SetSurface at 0x59c882 calls
   SetFFFogAlphaDisabled(1) when the Additive flag (0x10000) is set —
   sun, moon, stars, additive cloud sheets render unfogged. Without this
   gate the sun dimmed to fog color at horizon dusk/dawn instead of
   staying bright. Plumbed via new `uApplyFog` shader uniform driven by
   the existing SubMeshGpu.IsAdditive boolean (already set from
   TranslucencyKind.Additive at upload time).

User visually verified all three vs retail screenshots in Holtburg.
Tests: 1223 pass.
This commit is contained in:
Erik 2026-04-27 19:49:51 +02:00
parent 63b50c5291
commit 97fc1b51d8
4 changed files with 121 additions and 55 deletions
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33
src/AcDream.Core/Meshing/GfxObjSubMesh.cs
View file

@ -55,20 +55,25 @@ public sealed record GfxObjSubMesh(
public bool NeedsUvRepeat { get; init; } = false;
/// <summary>
/// <c>Surface.Translucency</c> float (0..1 — distinct from the
/// <c>Surface.Translucency</c> float (0..1) treated as an OPACITY
/// multiplier on fragment alpha. 1.0 = fully opaque (default for
/// non-Translucent surfaces). Distinct from the
/// <see cref="TranslucencyKind"/> classifier above, which buckets the
/// flag bits). Retail's <c>D3DPolyRender::SetSurface</c> at
/// <c>0x59c767</c> reads this when the <c>Translucent</c> (0x10) bit
/// is set on the surface and feeds it into the per-vertex alpha
/// (<c>curr_alpha</c>); the rasterizer then multiplies fragment alpha
/// by <c>(1 - translucency)</c> so the resulting opacity is
/// <c>1 - x</c>. ACViewer (<c>TextureCache.cs:142</c>) and WorldBuilder
/// (<c>ObjectMeshManager.cs:1115</c>) both use the same convention.
/// For the rain Surface 0x080000C5, <c>Translucency = 0.5</c> ⇒
/// opacity = 0.5; with the <c>(SrcAlpha, One)</c> additive blend the
/// rain streaks contribute at half intensity instead of full.
/// Defaults to 0.0 (fully opaque) so non-translucent surfaces render
/// through the normal lighting path without change.
/// flag bits. Retail's <c>D3DPolyRender::SetSurface</c> at
/// <c>0x59c7a6</c> (decomp lines 425255-425260) reads
/// <c>Surface.Translucency</c> when the <c>Translucent</c> (0x10) bit
/// is set, computes <c>curr_alpha = _ftol2(translucency × 255)</c>,
/// and writes that as vertex alpha — i.e. the dat's Translucency float
/// is used DIRECTLY as opacity, NOT inverted. ACViewer
/// (<c>TextureCache.cs:142</c>) and WorldBuilder
/// (<c>ObjectMeshManager.cs:1115</c>) both use <c>1 - translucency</c>
/// and are wrong by the same misread.
/// For the rain Surface 0x080000C5 (translucency=0.5): opacity = 0.5;
/// with the <c>(SrcAlpha, One)</c> additive blend the rain streaks
/// contribute at half intensity. For cloud surface 0x08000023
/// (translucency=0.25): opacity = 0.25 (matches retail's dim clouds).
/// Defaults to 1.0 (fully opaque) so non-Translucent surfaces render
/// at full opacity without change.
/// </summary>
public float SurfTranslucency { get; init; } = 0f;
public float SurfTranslucency { get; init; } = 1f;
}